Tag identification systems and methods

ABSTRACT

In a tag identification system, active tags are configured to emulate at least one passive tag so that existing passive identifications systems, such as passive radio-frequency (RFID) systems or near field communication (NFC) systems, can be retrofitted to operate with the active tag without having to make expensive hardware updates to the readers within the system. The active tag permits continuous monitoring of its movement or status, as may be desired, regardless of its proximity relative to the readers thereby allowing a system to capture more data regarding the tag&#39;s location and use. In addition, the active tag may be compatible with different passive identification systems thereby permitting it to operate with disparate passive identification systems and obviating the need of the user to carry multiple passive tags.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 14/165,119, entitled “Tag Identification Systemsand Methods” and filed on Jan. 27, 2014, which is incorporated herein byreference.

RELATED ART

Access to restricted areas is often controlled through the use ofpassive radio-frequency identification (“passive RFID”). In this regard,a passive RFID system generally comprises at least one active RFIDreader and at least one passive RFID tag. The active RFID readerperiodically transmits an interrogation signal for interrogating anyRFID tags that are within a short distance of the reader, such as a fewinches or, in some cases, in direct contact with the reader. If an RFIDtag is close enough to the reader, the interrogation signal energizes anelectrical circuit within the tag, which stores a tag identifier innon-volatile memory. When energized by the interrogation signal, theelectrical circuit modulates a signal with data defining the tagidentifier and possibly other information and wirelessly transmits thesignal to the reader. Based on the tag identifier, a controller incommunication with the reader determines whether to permit access to therestricted area. As an example, the controller may control the lockingstate of a door to the restricted area based on the identifier returnedby the tag. If the controller determines that the returned tagidentifier is valid, the controller automatically unlocks the door topermit entry to the restricted area. If not, the controller refrainsfrom unlocking the door thereby preventing unauthorized access to therestricted area.

While an RFID system can employ active tags, passive tags can beparticularly beneficial in many applications due to various factors. Inthis regard, passive tags do not generally require batteries, which havea finite life expectancy, and passive tags are typically less costlythan active tags. In addition, passive tags generally operate within ashort range, such as a few inches, of the reader that provides theinterrogation signal. Thus, when a tag is worn by a user, identificationof the tag generally confirms that the user is within a close proximityof the reader, which may be beneficial for various applications, such aswhen the system is used to control access to restricted areas. Forexample, when a tag is used to unlock a door so that a user may passthrough the door a restricted area, identification of the tag at thereader confirms that the user is physically at the door and ready topass through it, assuming that the reader is situated in a closeproximity to the door.

Once an RFID system is installed, updating the system can beproblematic. For example, the system may have expensive hardware (e.g.,readers) installed throughout a facility and replacing this hardware canbe burdensome and costly. In addition, a large organization may havemultiple RFID systems at one or more facilities where at least one ofthe RFID systems is incompatible with at least one of the other RFIDsystems. In such situation, employees or other personnel associated withthe organization may have multiple tags for the different RFID systems,and keeping track of multiple tags per user can be burdensome. In somecases, it may desirable to upgrade the disparate RFID systems to asingle system, but the costs of performing the upgrade can besignificant.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be better understood with reference to the followingdrawings. The elements of the drawings are not necessarily to scalerelative to each other, emphasis instead being placed upon clearlyillustrating the principles of the disclosure. Furthermore, likereference numerals designate corresponding parts throughout the severalviews.

FIG. 1 is a block diagram illustrating a conventional passive tagidentification system.

FIG. 2 is a block diagram illustrating exemplary controllers forcontrolling access to restricted areas depicted in FIG. 1.

FIG. 3 is a block diagram illustrating an exemplary embodiment of anactive tag identification system.

FIG. 4 is a block diagram illustrating an exemplary embodiment of awireless tracking system.

FIG. 5 is a block diagram illustrating an exemplary embodiment of anactive tag, such as is depicted by FIG. 3.

FIG. 6 is a graph illustrating power versus frequency for exemplarysignals received by a reader, such as is depicted by FIG. 3.

FIG. 7 is a graph illustrating power versus time for exemplary signalsreceived by a reader, such as is depicted by FIG. 3.

FIG. 8 is a graph illustrating power versus time for exemplary tagsignals transmitted by an active tag, such as is depicted by FIG. 5, inresponse to an interrogation signal from a reader, such as is depictedby FIG. 3.

FIG. 9 is a block diagram illustrating an exemplary embodiment of acommunication module, such as is depicted by FIG. 5.

DETAILED DESCRIPTION

The present disclosure generally pertains to tag identification systemsand methods. In one exemplary embodiment, an active tag is configured toemulate at least one passive tag so that existing passiveidentifications systems, such as passive radio-frequency (RFID) systemsor near field communication (NFC) systems, can be retrofitted to operatewith the active tag without having to make expensive hardware updates tothe readers within the system. The active tag permits continuousmonitoring of its movement or status, as may be desired, regardless ofits proximity relative to the readers thereby allowing the system tocapture more data regarding the tag's location and use. In addition, theactive tag may be compatible with different passive identificationsystems thereby obviating the need of the user to carry multiple passivetags.

FIG. 1 depicts a conventional passive RFID system 10 for controllingaccess to a plurality of restricted areas 12-14. In FIG. 1, therestricted areas 12-14 are rooms within a building, and the rooms can beaccessed from a hallway 18, as will be described in more detail below.The system 10 comprises a passive RFID tag 22 and a plurality of RFIDreaders 25-27 that are in communication with controllers 35-37,respectively, and each of the controllers 35-37 controls access to arespective restricted area. For example, as shown by FIGS. 1 and 2, thecontroller 35 is coupled to and controls a locking mechanism 31 of adoor 41 based on the reader 25, and the controller 35 normally keeps thelocking mechanism 31 in a locked state thereby preventing the door 41from opening. When the door 41 is closed and locked, users in thehallway 18 are prevented from passing through the doorway 51 to enterthe restricted area 12. When the controller 35 determines that access tothe restricted area 12 is permissible based on reader 25 or otherwise,the controller 35 transmits a control signal to the locking mechanism 31causing such mechanism 31 to transition to an unlocked state. When thelocking mechanism 31 is in an unlocked state, a user in the hallway 18may open the door 41 and enter into the restricted area 12 through thedoorway 51.

Referring to FIGS. 1 and 2, the controller 36 is coupled to and controlsa locking mechanism 32 of a door 42 based on reader 26, and thecontroller 37 is coupled to and controls a locking mechanism 42 of adoor 43 based on reader 27. Through techniques similar to thosedescribed above for the controller 35, the controller 36 controls thelocking mechanism 42 based on the reader 26 in order to control accessto the restricted area 13 through a doorway 52, and the controller 37controls the locking mechanism 43 based on the reader 27 in order tocontrol access to the restricted area 14 through a doorway 53. Forsimplicity of illustration, it will be assumed hereafter that thecontrollers 36 and 37, the readers 26 and 27, and the locking mechanisms42 and 43 are generally configured the same and operate the same ascontroller 35, the reader 25, and the locking mechanism 41,respectively, except as is otherwise described herein.

Note that the controllers 35-37 may be implemented in hardware,software, firmware, or any combination thereof. As an example, thecontrollers 35-37 may comprise analog circuitry and/or digital logic forperforming various functions, as will be described in more detailhereafter. If any controller includes software or firmware forperforming such functions, the controller may comprise memory forstoring the software or firmware, and the controller may also comprisean instruction execution apparatus, such as a digital signal processor(DSP) or central processing unit (CPU) for executing such software.

During operation, access to the restricted areas 12-14 is controlled viainteraction between the passive tag 22 and the readers 25-27. In thisregard, the tag 22 has an embedded passive electrical circuit (notshown) in which an identifier, referred to hereafter as “tagidentifier,” and possibly other information is stored in non-volatilememory. The tag 22 is not equipped with a power supply and generallyperforms no operations until it is interrogated by a signal from areader 25-27, as will be described in more detail hereafter.

Each reader 25-27 periodically transmits a wireless interrogation signalfor interrogating the tag 22 when it comes close, such as within a fewinches, of the respective reader 25-27. As an example, assume that auser desires to enter the restricted area 13 and begins walking towardthe door 42. The user typically carries the tag 22 as he or she walks.For example, the tag 22 may be attached to the clothing of the user orinserted into a pocket of the user's clothes. As the user beginsapproaching the door 42 and, therefore, the reader 26 (which ispositioned close to the door 42), the user may position the tag 22 suchthat it begins to receive the interrogation signals that areperiodically transmitted by the reader 26. Once the user positions thetag 22 close enough to the reader 26 such that the strength of aninterrogation signal received from the reader 26 exceeds a threshold,the passive electrical circuit in the tag 22 is energized by theinterrogation signal through inductive coupling such that the tag'selectrical circuit responds to the interrogation signal. Using theelectrical energy from the interrogation signal, the electrical circuitwirelessly transmits the tag identifier that is stored in the tag 22. Ifdesired, the electrical circuit may also transmit other informationstored in the tag 22.

The controller 26 typically stores or otherwise accesses a list of tagidentifiers that are valid for authorizing access to the restricted area13 corresponding to the reader 26. As an example, the controller 26 maybe in communication with a remote storage device (not shown), such as adatabase, for storing the list of valid tag identifiers and fromtime-to-time receive such list from the remote storage device. Thus, thelist can be updated remotely by a user without having to travel to thelocation of the reader 26 or controller 36. Also, note that it isunnecessary for the controller 26 to be located close to the reader 26.As an example, the controller 36 may be located remotely from the reader26 and the door 52 and communicate with the reader 26 and the lockingmechanism 32 through a network (not shown), such as a local area network(LAN) or wide area network (WAN).

When the reader 26 receives the tag identifier transmitted by the tag22, the reader 26 transmits such tag identifier to the controller 36,which compares the received tag identifier to the list of valid tagidentifiers stored for the reader 26. If the received tag identifierdoes not match any of the stored tag identifiers, the controller 36discards the received tag identifier without changing the state of thelocking mechanism 32, which as described above is normally in a lockedstate. However, if the received tag identifier matches one of the storedtag identifiers, the reader 26 transitions the locking mechanism 32 toan unlocked state thereby permitting the user of the tag 22 to open thedoor 42 or automatically opening the door 42 for the user. Thus, theuser is permitted to enter the restricted area 13 through the doorway52.

Note that the reader 26 may be configured such that several events mustoccur before it will provide the tag identifier to the controller 36 foranalysis in determining whether to transition the locking mechanism 32to an unlocked state. In this regard, when the reader 26 transmits aninterrogation signal, the reader 26 looks for a return signal that iswithin a specific frequency range (e.g., the frequency range of theinterrogation signal) and within a certain time window from transmissionof the interrogation signal. This time window corresponds to an amountof time that is expected for the interrogation signal to energize thetag 22 causing it to send a response signal back to the reader 26.Specifically, the time window is defined such that, if the tag 22 isclose to the reader 26, the response signal transmitted from the tag 22will be received in the window.

The received signal strength of the response signal should be within acertain range, referred to herein as “strength window,” in order to besuccessfully received by the reader 26. This strength window correspondsto the expected strength of the return signal when it is transmitted bythe tag 22 within a certain distance of the reader 26. In this regard,it is expected that the tag 22 will only transmit a return signal whenit is within a close proximity of the reader 26. If the tag 22 is toofar away, then the strength of the interrogation signal may be too lowto energize the tag 22. Thus, the circuitry of the reader 26 is designedto read a return signal having a received signal strength within thestrength window. If the received signal strength is above or below thiswindow, then the reader 26 may be unable to successfully detect suchsignal.

Thus, for a successful unlock event to occur, the reader 26 shouldreceive a signal that (1) is within a certain time window after thereader transmits an interrogation signal, (2) is within a certainfrequency range, referred to herein as “frequency window,” (3) has areceived signal strength within a certain strength window, and (4)defines a tag identifier that is included in the associated list ofvalid tag identifiers.

Note that the tag 22 may be similarly used to gain access to therestricted areas 12 and 14, assuming that its tag identifier is includedin the list of valid tag identifiers for the readers 25 and 27. However,the user may be selectively prevented from gaining access to any of therestricted areas 12-14 by defining the lists of valid tag identifierssuch that the identifier of the tag 22 is not in the list of valid tagidentifiers for the corresponding reader 25-27. For example, the usermay be prevented from accessing the restricted area 12 if the list ofvalid tag identifiers for the reader 25 does not include the identifierof the tag 22.

The system 10 has been described above in the context of a passive RFIDsystem that uses active readers 25-27 and passive tags 22 thatcommunicate within a short distance, such as a few inches. It ispossible to implement similar systems using other types of technologies.As an example, the readers 25-27 may be NFC devices.

FIG. 3 shows the identification system 10 after a wireless trackingsystem 100 is installed for tracking tags as they move. As shown byFIGS. 3 and 4, the system 100 comprises a plurality of nodes 101-104.FIG. 3 depicts four nodes 101-104 for simplicity, but the system 100 mayhave any number of nodes 101-104 in other embodiments. In one exemplaryembodiment, the system 100 and, in particular, the nodes 101-104implement a wireless mesh network 108, but other types of networks maybe implemented in other embodiments. Exemplary mesh networks aredescribed in commonly-assigned U.S. patent application Ser. No.12/114,566, entitled “Systems and Methods for Dynamically ConfiguringNode Behavior in a Sensor Network,” and filed on May 2, 2008, which isincorporated herein by reference, and commonly-assigned U.S. Pat. No.8,204,971, entitled “Systems and Methods for Dynamically ConfiguringNode Behavior in a Sensor Network” and filed on May 24, 2011, which isincorporated herein by reference.

Each node 101-104 is able to communicate with any of the other nodes101-104. In one exemplary embodiment, the nodes 101-104 communicateamong one another wirelessly, but it is possible for any of the nodes101-104 to communicate with any of the other nodes 101-104 over aconductive medium or otherwise. Messages may hop from node-to-node inorder to reach a destination. For example, in the exemplary embodimentshown by FIG. 4, the nodes 102-104 are within range of each other suchthat any of the nodes 102-104 can communicate directly with any of theother nodes 102-104. However, the node 101 is only within range of node104. The other nodes 102 and 103 can use the node 104 to communicatewith the node 101. Indeed, a message may hop through any number of nodesto reach its intended destination.

Referring to FIG. 4, at least one node 102 is coupled to a networkaccess device 111, which may have a network address and be a member ofthe network 108 formed by the nodes 101-104. The network access device111 is communicatively coupled to a network 116, such as a LAN or WAN,and interfaces messages between a protocol of the network 108 and aprotocol of network 116. As an example, in one embodiment, the network116 comprises the Internet, and messages compatible with the network 108are encapsulated in accordance with Transmission ControlProtocol/Internet Protocol for transmission across the network 116. Inanother embodiment, the network 116 comprises a LAN employing Ethernetprotocols, and messages compatible with the network 108 are encapsulatedin accordance with Ethernet protocol for transmission across the network116. In another exemplary embodiment, the network 116 may comprise apower-line network that communicates data messages across power lines,which simultaneously carry power signals (e.g., high-voltagetransmission signals).

As shown by FIG. 4, the network 116 is communicatively coupled to aserver 120, which will be described in more detail below. In oneexemplary embodiment, the server 120 has a network address and is amember of the network 108 formed by the nodes 101-104. Messagescompatible with network 108 are communicated between the server 120 andthe nodes 101-104 via the network 116 and network access device 111, aswill be described in more detail below. Note that server 120 may resideat a location close to at least one node 101-104 such that use of thenetwork 116 and network access device 111 to communicate between theserver 120 and the nodes 101-104 is unnecessary.

As shown by FIG. 3, the system 100 comprises at least one tag 122 thatis configured to wirelessly communicate with the nodes 101-104. The tag122 is a mobile device that may be attached or otherwise positioned on amobile asset (e.g., a person or object) in order to track movements ofthe asset, as will be described in more detail hereafter. In oneexemplary embodiment, the tag 122 is an active device having at leastone battery for powering electrical components of the tag 122, whichwill be described in more detail below. In addition, the tag 122 has anetwork address and is a member of the network 108, but it is preferablynot configured to route messages through the network 108. In thisregard, the tag 52 may transmit a network message to another node101-104 for communication of the message through the network 108, andmessages to be received and processed by the tag 122 may be communicatedto the tag 122 through the network 108. However, the tag 122 does notserve as a hop for messages and, thus, does not attempt to re-transmitmessages received by it. Preventing the tag 122 from performing routingfunctions helps to conserve the tag's power. In this regard, not onlyare the tag's functions reduced, but the tag may sleep from time-to-timewhile the nodes 101-104 remain operational for routing functions.However, if desired, the tag 122 may be configured to route messages inother embodiments.

FIG. 5 depicts an exemplary embodiment of the tag 122. As shown by FIG.5, the tag 122 comprises tag logic 152 for generally controlling theoperation of the tag 122, as will be described in more detail hereafter.The tag logic 152 can be implemented in software, hardware, firmware orany combination thereof. In the exemplary tag 122 illustrated by FIG. 5,the tag logic 152 is implemented in software and stored in memory 163 ofthe tag 122.

Note that the tag logic 152, when implemented in software, can be storedand transported on any computer-readable medium for use by or inconnection with an instruction execution apparatus that can fetch andexecute instructions. In the context of this document, a“computer-readable medium” can be any means that can contain or store acomputer program for use by or in connection with an instructionexecution apparatus.

The exemplary tag 122 depicted by FIG. 4 comprises at least oneconventional processing element 166, such as a digital signal processor(DSP) or a central processing unit (CPU), for executing instructionsstored in memory 163. The processing element 166 communicates to anddrives the other elements within the tag 122 via a local interface 172,which can include at least one bus. In addition, the tag 122 has acommunication module 175, which comprises an RF radio or other wirelesscommunication device for communicating wirelessly. The tag 122 also hasa power supply 177, such as one or more batteries, which provideelectrical power to the components of the tag 122. In addition, the tag122 has an input interface 181, such as a keypad, touchscreen, or dataport (e.g., a universal serial bus (USB) port) for receiving data, andan output interface 183, such as a display screen, speaker, vibrator,light source (e.g., a light emitting diode), or data port for providingoutput. The tag 122 also has a clock 188 for tracking time.

In one exemplary embodiment, the tag 122 is configured to communicatewith the nodes 101-104 so that the location of the tag 122 can betracked. In this regard, each node 101-104 transmits a wireless signal,referred to hereafter as “beacon,” that can be received by the tag'scommunication module 175. For illustrative purposes, assume that the tag122 receives the beacon transmitted by the node 101. The communicationmodule 175 is configured to measure a received signal strength of thebeacon, and the tag logic 152 is configured to transmit a message havinga value, referred to as a “received signal strength indicator” (RSSI),indicative of the received signal strength of the beacon. Such messageis communicated to the server 120 via the nodes 101-104, network accessdevice 111, and network 116. Based on the RSSI in the message, theserver 120 is configured to determine the location of the tag 122. Inthis regard, the RSSI is indicative of the distance of the tag 122 fromthe node 101, and the server 120 is configured to estimate the tag'sdistance from the node 101 based on the RSSI reported by the tag 122.

In some cases, the tag 122 may receive beacons from multiple nodes101-104 and report the RSSI to the server 120 for each beacon. In suchcase, the server 120 is configured to estimate the tag's distance frommultiple nodes 101-104 and use triangulation, trilateration, or someother location-determining algorithm to determine the tag's preciselocation. In other embodiments, other techniques may be used todetermine the tag's location.

Note that the location-determining algorithm may utilize informationindicative of various points of references in order to calculate thelocation of the tag 122. As shown by FIG. 4, the server 120 may storelocation data 121 indicative of the locations of such points ofreference. As an example, the location data 121 may include the locationcoordinates of each of the nodes 101-104. Knowing the locations of thenodes 101-104 and the estimated distances of the tag 122 from multiplenodes 101-104, it is possible to calculate the location coordinates ofthe tag 122. The location data 121 may indicate the locations of otherpoints, such as the locations of the readers 25-27 and restricted areas12-14, for example.

The server 120 is configured to store the tag's locations over time todefine a history of the tag's movement. Such history can be lateranalyzed to determine the tag's location at any given time duringtracking. In one exemplary embodiment, the server 120 is configured toenforce various rules and take certain actions based on the tag'slocation. As an example, the server 120 may store data indicating whichareas of a facility or other location the tag 122 is authorized toaccess. If the server 120 determines that the tag 122 is currently in anunauthorized area, the server 120 may generate an alarm. The server 120may also transmit a message to the tag 122 for causing the tag toprovide an output for warning the user of the tag 122. Various othertypes of actions may be taken in other embodiments based on the tag'slocation.

If desired, functions described herein as being performed by the server120 may be performed by the tag 122. As an example, the tag logic 152may be configured to estimate its distances from the nodes 101-104 basedon the received signal strengths of beacons or otherwise and to thendetermine the tag's location based on such distances. In suchembodiment, the location data 121 described above may be stored at thetag 122, as shown by FIG. 5.

In one exemplary embodiment, the tag 122 is configured to communicatewith the readers 25-27 in order to provide the user of the tag 122access to restricted areas 12-14 for which he or she is authorized toaccess. Thus, the active tag 122 may be used as a replacement for thepassive tag 22 described above with reference to FIG. 1. Further, in oneexemplary embodiment, the use of the active tag 122 in lieu of theconventional passive tag 22 is transparent to the readers 25-27 andcontrollers 35-37. That is, the tag 122 is configured to communicatewith the readers 25-27 such that they are unaware that they are notcommunicating with a conventional passive tag 22. Thus, there is no needto reconfigure or update the readers 25-27 or controllers 35-37 in orderto enable communication with the tag 122, and the readers 25-27 mayoperate in conjunction with the tag 122, as will be described in moredetail hereafter.

For example, as described above, the readers 25-27 may be configuredsuch that a successful read occurs when a reader 25-27 receives a signalthat is within a certain frequency window, a certain signal strengthwindow, and a certain time window relative to a transmission of aninterrogation signal by the reader. Such windows are illustrated inFIGS. 6 and 7.

In this regard, FIG. 6 shows an exemplary graph of power versusfrequency when the reader 26 is receiving three narrowband signals,represented by curves 191-193, respectively. Referring to FIG. 6, thereader 26 may be configured to filter energy outside of the frequencywindow 194, which corresponds to a passband centered around thefrequency (f_(TX)) of the interrogation signal transmitted by the reader26. Thus, the reader 26 suppresses the signals represented by curves 191and 193 and only processes the signal represented by curve 192 withinthe reader's passband. Accordingly, if the tag 122 transmits a signalhaving a frequency outside of the frequency window 194 (i.e., less thana lower frequency threshold (TH_(L)) or greater than an upper frequencythreshold (TH_(U))), then the signal will not be successfully receivedby the reader 26.

FIG. 7 shows an exemplary graph of power versus time when the reader isreceiving two signals, represented by curves 196 and 197, respectively.Referring to FIG. 7, the reader 26 is configured to transmit twosuccessive interrogation signals at times to and t₃, respectively.Further, the reader 26 may be configured to begin looking for a responsesignal for the first interrogation signal at some time t₁ aftertransmission of this interrogation signal and stop looking for suchresponse signal after some time t₂ after transmission of the firstinterrogation signal, thereby defining a time window 198 between t₁ andt₂. Thus, since the signal represented by curve 196 is within this timewindow 198, such signal should be successfully received by the reader26, assuming that its signal strength and frequency are within theappropriate signal strength window and frequency window, respectively.However, the signal represented by the curve 197 is received after thereader 26 stops looking for a response to the first interrogation signal(i.e., after t₂) and before the reader begins looking for a response tothe second interrogation signal (i.e., before t₃). Thus, this signal isnot successfully received by the reader 26.

FIG. 7 also shows a signal strength window 190 having an upper powerthreshold P_(U) and a lower power threshold P_(L). The lower thresholdP_(L) may be set just above the expected noise floor in an effort tomitigate the effects of noise. If received energy is below P_(L), thereader 26 refrains from processing the energy. If the power of areceived signal exceeds P_(U), then components of the reader 26 maysaturate such that the reader 26 is unable to detect modulation of thereceived signal. In FIG. 7, the signal represented by the curve 196should be successfully received by the reader 26 since it is within anexpected time window between t₁ and t₂ and since its signal strength iswithin the signal strength window 190 between P_(L) and P_(U).

In one exemplary embodiment, the tag 122 is configured to transmit asignal that is within the frequency window, signal strength window, andtime window expected by a reader 25-27 for a response to itsinterrogation signal. As an example, assume that, as the tag 122 isapproaching the reader 26, the communication module 175 of the tag 122receives an interrogation signal from the reader 26. In response to theinterrogation signal, the tag logic 152 is configured to transmit to thereader 26 a signal, referred to hereafter as “tag signal.” The timingand frequency of the tag signal are controlled based on theinterrogation signal such that they are respectively within the timewindow and the frequency window expected by the reader 26 for aresponse.

In particular, with respect to the timing window, the tag logic 152 isconfigured to track time based on the clock 188 after receiving aninterrogation signal. Once the elapsed time from reception of theinterrogation signal exceeds a predefined threshold, the tag logic 152is configured to wirelessly transmit the tag signal via thecommunication module 175. The threshold is preferably set such that thereader 26 receives the tag signal within the expected time window thatis associated with the interrogation signal. As an example, when theconventional passive tag 22 receives an interrogation signal from thereader 26, there is a finite delay that occurs before the passive tag 22transmits a response signal. In particular, there is a delay as theelectrical circuit within the passive tag 22 energizes and then performsprocessing in order to transmit the response signal. In FIG. 7, suchdelay may correspond approximately to the difference between t₁ and to.For the active tag 122 illustrated by FIG. 5, the tag logic 152 mayaccount for such delay by transmitting the tag signal at a similar delayfrom the time of reception of the interrogation signal. Thus, in FIG. 7,the tag logic 152 may transmit the tag signal at about or shortly aftertime t₁.

In addition, the tag logic 152 may also account for the location of thetag 122 in determining when to transmit the tag signal. In this regard,as described above, it is generally expected for the passive tag 22 tobe close to the reader 26 (e.g., a few inches depending on theapplication or identification technology employed) when responding to aninterrogation signal from the reader 26. However, the active tag 122 maynot rely on the energy from the interrogation signal in order totransmit the tag signal and, thus, may be located a greater distanceaway when responding to the interrogation signal. Generally when the tag122 is located a greater distance away from the reader 26, it takeslonger for the tag signal to reach the reader 26. Based on the tag'sdistance from the reader 26 and the expected delay associated with thepassive tag 22, the tag logic 152 may be configured to adjust the timingof the tag signal's transmission such that it arrives at the reader 26at approximately the same time that a response signal from the passivetag 22 would have arrived at the reader 26 if the passive tag 22 wouldhave been close enough to the reader 26 in order to respond to theinterrogation signal.

Note that there are various methodologies that may be used to ensurethat the tag signal arrives at the reader 26 within the expected timingwindow (e.g., within a certain time period after transmission of aninterrogation signal). In one exemplary embodiment, the expected delayfor the conventional passive tag 22 is known such that the appropriateamount of delay to be used by the active tag 122 can be calculated. Insuch embodiment, information indicative of the desired delay for thereader 26 is downloaded to the tag 122 and stored in memory 163 as data199, referred to herein as “reader data.” As an example, the reader data199 may include a time threshold corresponding to a delay that is to beused by the tag logic 152 for the reader 26. As described above, when aninterrogation signal from the reader 26 is received, the tag logic 152begins to track time and transmits the tag signal after the foregoingtime threshold is exceeded.

In another example, the reader data 199 may define a table of timethresholds respectively correlated with distance values. In such anembodiment, based on the tag's current distance from the reader, the taglogic 152 looks up the time threshold that is correlated with the tag'sapproximate distance from the reader 26. Thus, the time thresholdselected is based on the tag's distance from the reader 26. The taglogic 152 may also calculate the time threshold or desired delay on thefly.

In other embodiments, the expected delay of the conventional passive tag22 may be unknown or the reader data 199 described above may beunavailable. In such case, the tag logic 152 may be unaware of theprecise delay to use in order to ensure that the tag signal istransmitted at the appropriate time to arrive at the reader 26 duringthe expected time window. In such an embodiment, the tag logic 152 maybe configured to repetitively transmit the tag signal after receivingthe interrogation signal such that at least one of the tag signals islikely to be received by the reader 26 during the expected time window.

For example, after receiving an interrogation signal from the reader 26,the tag logic 152 may be configured to periodically transmit the tagsignal for a duration, referred to herein as “response period,” that issufficiently long so that it is likely that the periodic transmissionscontinue for a time beyond the expected time window within a desiredmargin of error. Further, the transmission rate of the tag signals issufficiently fast (or, in other words, the time between successive tagsignals is sufficiently short) so that it is likely that at least one ofthe tag signals is received during the expected time window. In oneexemplary embodiment, the tag logic 152 is configured to transmit thetag signal every few milliseconds during the response period afterdetection of an interrogation signal.

As an example, referring to FIG. 8, assume that the tag logic 152detects an interrogation signal at time to. In response, for a durationof a response period extending from t₀ to t₄, the tag logic 152periodically transmits four tag signals, represented as curves 202-204,respectively. Shortening the time between successive tag signals duringthe response period may increase the likelihood that at least one of thetag signals will be within the expected time window for the reader 26.However, shortening the time between successive tag signals also mayincrease the number of tag signals that will fit within the responseperiod, thereby requiring more power to transmit the additional tagsignals. Accordingly, the duration of the response period and the rateof the tag signal transmissions within the response period may beoptimized based on desired performance considerations.

Note that over time, the tag logic 152 may automatically learn a moresuitable duration for the response period. As the tag logic 152 isrepetitively transmitting tag signals during the response period afteran interrogation signal is received from the reader 26, the tag logic152 may listen for a new interrogation signal from the same reader 26.Once a new interrogation signal is received, the tag logic 152 is awarethat the time window for the previous interrogation signal likelyoccurred prior to reception of the new interrogation signal. Thus, theduration of the response period for future responses may be shortenedsuch that it is less than the time between the successive interrogationsignals. Therefore, when the tag 122 receives an interrogation from thesame reader 26 in the future, the tag logic 152 responds by repetitivelytransmitting tag signals for a shorter duration, thereby conservingpower. Specifically, when an interrogation signal is received, the tag122 should stop transmitting tag signals at least by the time the nextinterrogation signal is transmitted from the same reader.

As an example, referring to FIG. 8, assume that the tag logic 152 isconfigured to transmit four tag signals in the response period as shown,although it is possible for the tag logic 152 to be configured totransmit any number of tag signals in the response period in otherembodiments. Further assume that at time t_(int), between t₃ and t₄, thetag 122 receives another interrogation signal. In such embodiment, thetag logic 152 may determine that the response period can be shortened tobe between t₀ and t_(int) such that, in the future, the tag logic 152only transmits three tag signals in response to an interrogation signal.

In another embodiment, assume that t_(int) occurs after the last tagsignal of the response period (i.e., after t₄). In such case, if thedelay between the end of the response period (i.e., t₄) and the nextinterrogation signal is greater than a predefined threshold, the taglogic 152 is configured to increase the duration of the response period.The tag logic 152 may also increase or alternatively not change thetransmission rate of the tag signals during the response period.Depending on whether and to the extent that the transmission rate isincreased, the tag logic 152 may thereafter transmit a higher number oftag signals in the response period and/or spread the tag signals apartmore in time.

In addition, the tag 122 may control the rate at which it transmits thetag signals during the response period based on the rate at which theinterrogation signals are received regardless of whether the duration ofthe response period is changed. For example, if the rate of theinterrogation signal is relatively high (e.g., the time betweensuccessive interrogation signals is long), the tag logic 152 may beconfigured to control the rate of the tag signals such that the timebetween successive tag signals is longer. Conversely, if the rate of theinterrogation signals is relatively low (e.g., the time betweensuccessive interrogation signals is short), the tag logic 152 may beconfigured to control the rate of the tag signals such that the timebetween successive tag signals is shorter. In other words, the tag logic152 may increase the rate of the tag signals when it is determined thatthe reader 26 is transmitting interrogation signals at a faster rate,and the tag logic 152 may decrease the rate of the tag signals when itis determined that the reader 26 is transmitting interrogation signalsat a slower rate.

In another exemplary embodiment, the tag logic 152 is configured toautomatically learn the appropriate delay between reception of theinterrogation signal and transmission of the tag signal in order toensure that the tag signal is received during the expected time windowwithout having to repetitively transmit the tag signal. For example, thetag logic 152 may adjust the delay by trial and error until anacceptable delay is found.

In this regard, when an interrogation signal is received by thecommunication module 175, the tag logic 152 is initially configured totransmit the tag signal after a certain delay. The selection of suchdelay could be predefined or randomly selected within a predefinedrange. The tag logic 152 then determines whether the delay isacceptable. Such determination can be performed in a variety of ways.For example, if the reader 26 transmits an acknowledgment of receiving atag signal within the time window, the tag logic 152 may determinewhether the delay is acceptable based on whether an acknowledgement isreceived.

In another embodiment, the determination about whether the delay isacceptable is made based on whether the controller 36 permits access tothe corresponding restricted area 13. For example, in some embodiments,the controller 36 may be configured to inform the server 120 when it istransitioning the locking mechanism 32 to an unlocked state or isotherwise permitting access to the restricted area 13. In such case, theserver 120 transmits a message to the tag 122 indicating that the door42 has been unlocked, and the tag logic 152 determines that the delayused to transmit the tag signal is acceptable. In another embodiment,such as when the server 120 and controller 36 are not configured forcommunication with each other, the server 120 monitors the location ofthe tag 122 to determine whether the controller 36 permits access to therestricted area 13 in response to the tag signal from the tag 122.

In this regard, when the tag logic 152 transmits a tag signal to thereader 26, the tag logic 152 may be configured to send a messageindicative of such event to the server 120. In response, the server 120monitors the location of the tag 122 for determining whether the tag 122moves toward or through the doorway 52 corresponding to the reader 26,such as when the user opens the door 42 and begins to walk through thedoorway 52. If so, then it can be assumed that the controller 36 hasunlocked the door 42 in response to the tag signal. In such case, theserver 120 transmits a message to the tag 122 indicating that the door42 has been unlocked, and the tag logic 152 determines that the delayused to transmit the tag signal is acceptable. After determining thatthe delay is acceptable, the tag logic 152 stores a value indicative ofsuch delay and retrieves this value the next time that the tag 122 is torespond to an interrogation signal from the same reader 26 so that thesame delay is thereafter used in responding to interrogation signalsfrom this reader 26.

However, if a predefined amount of time elapses after transmission ofthe tag signal without a determination being made that theaforementioned delay is acceptable, then the tag logic 152 determinesthat the delay used to transmit the tag signal is unacceptable for thereader 26. In such case, the tag logic 152 uses a different delay forthe next interrogation signal from the same reader 26. This process maybe repeated until an acceptable delay is found.

As described above, for the reader 26 to perform a successful read ofthe tag signal, the tag signal should be transmitted at a frequency thatis in the reader's frequency window. In one exemplary embodiment, thetag logic 152 assumes that the tag signal should be at the samefrequency as the interrogation signal and, therefore, transmits the tagsignal at such frequency. Note that there are a variety of ways that thetag logic 152 can determine the frequency of the interrogation signal.For example, the reader data 199 may indicate the transmission frequencyof each reader 25-27, and the tag logic 152 may determine the frequencyof the reader 26 based on the data 199.

In addition, the tag logic 152 may dynamically determine the frequencyused by the reader 26. In this regard, FIG. 9 shows an exemplaryembodiment of the communication module 175. As shown by FIG. 9, thecommunication module 175 comprises an antenna 205, an adjustable filter208, a receiver 209, and a transmitter 210. The adjustable filter 208 iscoupled between the receiver 209 and the antenna and has a passband thatis adjustable based on a control signal from the tag logic 152. When thetransmission frequency of the reader 26 is known (e.g., indicated by thereader data 199), the tag logic 152 is configured to control theadjustable filter 200 such that the frequency of the interrogationsignal is within the passband of the filter 208. Thus, when theinterrogation signal is received by the antenna 205, the signal passesthrough the adjustable filter 208 to the receiver 209. The tag logic 152also controls the transmission frequency of the transmitter 210 suchthat it is within this same passband. Thus, the communication module 175transmits and receives at approximately the same frequency, though it ispossible for the module 175 to transmit and receive at differentfrequencies in other embodiments. If the reader 26 transmits andreceives at the same frequency, then setting the transmission frequencyto the expected frequency of the interrogation signal should ensure thattransmitter 210 transmits the tag signal within the appropriatefrequency window for successful reception by the reader 26.

In some embodiments, the frequency of the interrogation signal may beunknown to the tag logic 152. In such embodiment, the tag logic 152 maybe configured to automatically discover the frequency of theinterrogation signal. As an example, the tag logic 152 may be configuredto cycle through various frequency bands to find a band for which theinterrogation signal is successfully received. In this regard, the taglogic 152 controls the adjustable filter 208 such that it is tuned to aspecific frequency band and then waits for a predefined amount of timeto determine whether the receiver 209 receives an interrogation signal.

After setting the adjustable filter 208 to a specific passband, the taglogic 152 analyzes the energy passing through the filter to determinewhether it defines an interrogation signal. If so, the tag logic 152sets the transmission frequency of the transmitter 210 within this samefrequency band and thereafter communicates with the reader 26 withinthis frequency. However, if the tag logic 152 does not detect aninterrogation signal after the predefined amount of time, the logic 152adjusts the filter 208 such that its passband is changed and repeats theaforementioned process until the frequency band of the interrogationsignal is found.

As described above, for the reader 26 to perform a successful read ofthe tag signal, the received signal strength of the tag signal should bein the reader's strength window. In one embodiment, the reader data 199specifies a transmit signal strength for the tag signal, and the taglogic 152 causes the communication module 175 to transmit the tag signalat this indicated signal strength. In another embodiment, the tag logic152 may find the appropriate signal strength via trial and error similarto the trial-and-error techniques described above for finding theappropriate delay between reception of the interrogation signal andtransmission of the tag signal. In this regard, the tag logic 152 may beconfigured to cause the communication module 175 to transmit the tagsignal at a specific power and then determine whether this transmitpower is acceptable via the same techniques described above fordetermining whether the aforementioned delay is acceptable. That is, ifthe reader 26 successfully receives the tag signal such that the door 42is unlocked, then the tag logic 152 determines that the transmit poweris acceptable. In such case, the tag logic 152 causes the communicationmodule 175 to transmit tag signals to the reader 26 at this same powerin the future. However, if the transmit power is determined to beunacceptable, then the tag logic 152 is configured to try a differenttransmit power the next time the tag signal is transmitted.

In addition, the tag logic 152 may also account for the location of thetag 122 in determining the appropriate transmit power for the tagsignal. In this regard, as described above, it is generally expected forthe passive tag 22 to be close to the reader 26 when responding to aninterrogation signal from the reader 26. However, the active tag 122 maynot rely on the energy from the interrogation signal in order totransmit the tag signal and, thus, may be located a greater distanceaway when responding to the interrogation signal. Generally, when thetag 122 is located a greater distance away from the reader 26, moresignal attenuation occurs before the tag signal reaches the reader 26such that the tag signal arrives at the reader 26 with a lower signalstrength. Based on the tag's distance from the reader 26, the tag logic152 may be configured to adjust the tag signal's transmit power suchthat it arrives at the reader 26 with a signal strength within thestrength window.

In one exemplary embodiment, the reader data 199 may define a table oftransmit power values correlated with distance values. In such anembodiment, the tag logic 152 determines the tag's distance from thereader 26. As described above, the server 120 may determine the tag'slocation and send a message to the tag 122 indicative of the tag'slocation or its distance from the reader 26. Based on the tag's currentdistance from the reader 26, the tag logic 152 looks up the transmitpower value that is correlated in the table with the tag's approximatedistance from the reader 26, and the tag logic 152 then controls thetransmitter 210 such that the power of the tag signal is in accordancewith that indicated by the correlated power value. Thus, the transmitpower of the tag signal is selected based on the tag's distance from thereader 26. The tag logic 152 may also calculate the transmit power onthe fly based on the tag's distance from the reader 26.

It is also possible for the transmit power to be selected based on thereceived signal strength of the interrogation signal. In this regard,the communication module 175 is configured to measure the signalstrength of the received interrogation signal and to transmit a valueindicative of this measured signal strength to the tag logic 152. Basedon this value, the tag logic 152 controls the communication module 175such that the transmit power of the tag signal is about the same as thereceived power of the interrogation signal, though it is possible forthe tag signal have a higher or lower power.

In one exemplary embodiment, the tag logic 152 uses the location data121 and the knowledge of the tag's location when the interrogationsignal is received in order to determine the distance between the tag122 and the reader 26. Knowing such distance and the received signalstrength of the interrogation signal, the tag logic 152 estimates thetransmit power of the interrogation signal (i.e., the power of theinterrogation signal at the time of transmission from the reader 26).Based on the distance between the tag 122 and the reader 26, the taglogic 152 then estimates a transmit power for the tag 122 that willlikely cause the tag signal to arrive at the reader 26 with a certainsignal strength that is based on the estimated transmit power of theinterrogation signal. For example, the tag logic 152 may estimate thetransmit power that will likely result in the tag signal arriving at thereader 26 with about the same or slightly less signal strength as thatof the interrogation signal at the time of transmission from the reader26. The tag logic 152 then controls the communication module 175 suchthat the tag signal is transmitted with the estimated transmit power.

As described above, for the controller 36 to grant access to itscorresponding restricted area 13, the tag signal should define a validtag identifier. In one embodiment, the reader data 199 specifies a tagidentifier for the tag signal, and the tag logic 152 causes thecommunication module 175 to modulate the tag signal with the tagidentifier. In another embodiment, the tag logic 152 may find a validtag identifier via trial and error similar to the trial-and-errortechniques described above for finding the appropriate delay fortransmitting the tag signal in response to an interrogation signal. Inthis regard, the tag logic 152 may cause the communication module 175 tomodulate the tag signal with a specific tag identifier and, aftertransmitting the modulated tag signal to the reader 26, determinewhether this tag identifier is valid via the same techniques describedabove for determining whether the aforementioned delay was acceptable.That is, if the reader 26 successfully receives the tag signal, and thecontroller 26 unlocks the door 42 in response, then the tag logic 152determines that the tag identifier is valid. In such case, the tag logic152 causes the communication module 175 to continue using this tagidentifier to modulate tag signals in the future. However, if the tagidentifier is determined not to be valid, then the tag logic 152 isconfigured to try a different tag identifier the next time the tagsignal is transmitted. This process may be repeated until a valid tagidentifier is found.

After a door has been unlocked to permit access to a restricted area inresponse to a tag identifier from the tag 122, it may be desirable toprevent the tag 122 from causing an unlock of the same door for a periodof time. In one exemplary embodiment, after a successful unlock event,the tag 122 is prevented from initiating a new unlock event for the samedoor for a predefined period of time. In this regard, as describedabove, there are various techniques that can be used to determine when atag signal has been successfully communicated to a reader 26. Once thetag logic 152 determines that a tag signal has been successfullycommunicated to the reader 26, the tag logic 152 is configured torefrain from responding to an interrogation signal from the same reader26 for a predefined time period, thereby preventing multiple unlocks ofthe same door 42 during the time period.

When exiting a control point, often there is no need for secure egress.However, when employing an embodiment of the disclosure, now even onegress it can be determined whether or not a person having access isgranted egress. This may be of particular importance in certainsituations where accountability is needed. For instance, in a highrisesituation, it may be possible to program a backdoor into the passivesystems so that firemen could have a universal access key that broadcasta plurality of keys to gain access to a bank of elevators based on thetop 10 manufacturers of controlled access companies in the city. Itcould then prioritize that key for future passive queries until it nolonger works. At which time it would try other keys. Additionally theactive device could also hold that key until manually reset after, forexample, a fire, whereby it would revert back to a preordered list ofkeys.

Emergency access and accountable egress also becomes important forrescue personnel when trying to control a highrise. By being able tocontrol the egress point, one can determine who is still inside thecontrolled area. In another embodiment of the disclosure, passivesystems can be used to determine crude locations of active tag wearersby merely monitoring which doors are being unlocked as people pass thru.It would also be useful for systemic clearing of a building so thatprogress thru the building could be tracked.

In another security-related embodiment, active tags 122 of lawenforcement could be quickly configured to emulate a set of passive tags22 before going into a location. For example, a plurality of tagidentifiers of passive tags 22 may be input into an active tag 122 sothat this active tag 122 can be used to access all of the restrictedareas capable of being accessed via any of the plurality of passive tags22. In such embodiment, upon receiving an interrogation signal from areader, the active tag 122 may be configured to cycle through the storedtag identifiers by transmitting each tag identifier to the reader untilthe reader recognizes at least one of the tag identifiers such thataccess to the corresponding restricted area is granted. Exemplarytechniques for determining when access to a restricted area is grantedare described in more detail hereinabove. Since the active tag 122 canbe used to access all of the restricted areas accessible via any of thepassive tags 22, it would be unnecessary for janitors or other onsitepersonnel to accompany law enforcement personnel when searching abuilding.

Furthermore the active tags 122 could be configured so that certain tags122 only work during certain hours of the day. For instance, in ashopping environment, it is possible to restrict a controller fromallowing access to a restricted area outside regular hours, but it isalso possible to increase the security of a location so that an activetag 122 only broadcasts tag signals during certain hours. These hoursmay be coordinated with the shifts that the user of the active tag 122works.

As an example, FIG. 4 depicts an exemplary embodiment in which theserver 120 stores schedule data 206 indicative of a predefined schedulefor the tag 122. Such schedule data 206 may indicate time periods inwhich the tag 122 is to be disabled from responding to interrogationsignals. As an example, the time periods may correspond to when the userof the tag 122 is scheduled to be off work or other time period in whichit might be desirable for the tag 122 to be disabled. When the server120 determines that the tag 122 is to be disabled based on such data206, the server 120 transmits to the tag 122 a message for disabling thetag 122 from responding to interrogation signals. In response, the taglogic 152 is configured to refrain from responding to interrogationsignal until it receives another message indicating that it is enabled.In this regard, once the server 120 determines based on the scheduledata 206 that the tag 122 should no longer be disabled, the server 120transmits to the tag 122 a message for enabling the tag 122 to respondto interrogation signals. In response, the tag logic 152 is configuredto begin responding to interrogation signals. Selective disabling of thetag 122, as described above, may not only be useful in enhancing thesecurity of restricted areas but also may be useful for conserving thepower resources of the tag 122. That is, the tag 122 does not wastepower trying to respond to interrogation signals when it should not bein use by its associated user.

As described above, the server 120 may be configured to monitor thelocation of the tag 122 as it travels. In one exemplary embodiment, theserver 120 is configured to control whether the tag 122 responds to oneor more interrogation signals based on the tag's tracked movements. Forexample, assume that the user carrying the tag 122 is authorized toaccess the restricted area 13 but not the restricted area 12. Such rulesmay be indicated by the location data 121 stored at the server 120 sothat the server 120 is aware of which restricted areas the user ispermitted to access.

The server 120 may be configured to enable the tag 122 to respond tointerrogation signals only when the tag 122 is within certain areas,referred to herein as “interrogation-enabled zones,” such as apredefined distance of a reader that is controlling access to arestricted area for which the user is permitted to access. Forillustrative purposes, FIG. 3 shows two exemplary interrogation-enabledzones 213 and 214 for the tag 122. The interrogation-enabled zone 213corresponds to the reader 26 in that the tag 122 likely receivesinterrogation signals from the reader 26 when in the zone 213, and theinterrogation enabled zone 214 corresponds to the reader 27 in that thetag 122 likely receives interrogation signals from the reader 27 when inthe zone 214. In the embodiment shown by FIG. 3, assume that the tagidentifier of the tag 122 is valid for readers 26 and 27, which areconfigured to permit access to the restricted areas 13 and 14,respectively, in response to such tag identifier. The tag 122 maycommunicate with the reader 26 when in the interrogation-enabled zone213 and may communicate with the reader 27 when in theinterrogation-enabled zone 214, as will be described in more detailbelow. Further assume that the tag identifier of the tag 122 is notvalid for the reader 25. Thus, there is no need for the tag 122 tocommunicate with the reader 25 for the purpose of attempting to accessthe restricted area 12 associated with the reader 25. In otherembodiments, any number of interrogation-enabled zones corresponding toany number of readers may be implemented, and such zones may be locatedat any location as may be desired. Note it is unnecessary for the sameinterrogation-enabled zones to be used for different tags 122.

Initially, when the tag 122 is far from the reader 26 and not within anyinterrogation-enabled zone 213 or 124 for the tag 122, the tag 122 maybe disabled from responding to interrogation signals. Areas outside ofinterrogation-enabled zones shall be referred to herein as“Interrogation-disabled zones.” As the tag 122 approaches the reader 26and comes within the interrogation-enabled zone 213 associated with thisreader 26, the server 120 is configured to detect this event and to thenenable the tag 122 to respond to interrogation signals. As an example,the server 120 may transmit a message to the tag 122 instructing it torespond to interrogation signals. Later if the tag 122 leaves suchinterrogation-enabled zone 213, thereby entering aninterrogation-disabled zone, the server 120 may transmit another messagefor disabling the tag 122 from responding to interrogation signals.Preferably, the tag 122 is enabled to respond to interrogation signalsonly when it is within one of its interrogation-enabled zones 213 or214.

In any event, once the tag 122 enters the interrogation-enabled zone 213for the reader 26, the server 120 enables the tag 122 to respond tointerrogation signals, as described above. Thereafter, when the tag 122receives an interrogation signal from the reader 26, the tag 122attempts to respond to the interrogation signal by transmitting one ormore tag signals to the reader 26. In response to such a tag signal, thecontroller 36 may unlock the locking mechanism 32, thereby allowing theuser to enter the restricted area 13.

Note that the control of the tag 122, as described above, may preventthe tag 122 from attempting to respond to an interrogation signal fromthe reader 25 as the tag 122 passes by the reader 25. In this regard, asthe tag 122 travels, it may come sufficiently close to the reader 25such that it receives an interrogation signal from this reader 25. Ifthe tag 122 has not yet entered one of its interrogation-enabled zones,such as the interrogation-enabled zone 213 associated with the reader26, then the tag 122 is disabled from responding to interrogationsignals when it receives the interrogation signal from the reader 25.Thus, the tag 122 does not attempt to respond to this interrogationsignal.

Preventing the tag 122 from responding to this interrogation signalshould not present any operational problems since, in the currentexample, the user of the tag 122 is not permitted to access therestricted area 12 corresponding to the reader 25. That is, even if thetag 122 responds to the interrogation signal from the reader 25, thecontroller 35 in the current example should not recognize the tag'sidentifier as valid and, therefore, should simply discard the tag'sresponse without taking any action to permit access to the area 12.Thus, there is no adverse consequence to preventing the tag 122 fromresponding to the interrogation signal transmitted by the reader 25.

However, preventing the tag 122 from responding to interrogation signalswhile outside of the tag's interrogation-enabled zones may have severalbenefits. For example, the tag 122 is prevented from wasting powerattempting to respond to readers, such as reader 25 in the exampledescribed above, corresponding to restricted areas that the user is notauthorized to access. Also, inadvertent unlocking of doors for variousrestricted areas may also be prevented. As an example, when traveling tothe door 43, the user may walk close enough to the reader 26 to receivean interrogation signal from the reader 26 but not close enough to enterthe corresponding interrogation-enabled zone 213. In such case, the tag122 is not enabled to respond to the interrogation signal, therebypreventing the tag 122 from unlocking the door 42 as it passes. Thus,the user is prevented from unintentionally unlocking the door 42 andpotentially compromising the security of the restricted area 13 when heor she passes the reader 26.

The interrogation-enabled zones 213 and 214 may be defined such that thetag 122 is disabled from responding to interrogation signals once itenters into a restricted area. For example, the interrogation-enabledzone 213 associated with the reader 26 may have a boundary at or closeto the threshold of door 42 such that the user leaves theinterrogation-enabled zone 213, thereby entering aninterrogation-disabled zone, when he or she passes through the doorway52. Therefore, after the user has passed through the doorway 52 andentered the restricted area 13, the tag 122 should be outside of any ofits interrogation-enabled zones and, thus, disabled from responding tointerrogation signals. This prevents the tag 122 from unlocking the door42 while it is in restricted area 13. Otherwise, the user couldunintentionally cause the door 42 to unlock (potentially compromisingthe security of the restricted area 13) by walking close enough to thereader 26 such that the tag 122 communicates with the reader 26. Toleave the restricted area 13, the user may manually transition thelocking mechanism 32 to an unlocked state.

In a further effort to prevent inadvertent door unlocks, the tag logic152 may be configured to respond to an interrogation signal only if thereceived signal strength of the interrogation signal exceeds apredefined threshold. Further, the tag logic 152 may be configured tocompare the RSSIs of multiple interrogation signals being received bythe tag 122 during a given time period and identify the interrogationsignal having the highest RSSI, which indicates that this interrogationsignal is from the closest reader assuming that all of the readers aretransmitting about the same power. If the highest RSSI is above thepredefined threshold, then the tag logic 152 is configured to respond tothe interrogation signal, referred to hereafter as “strongestinterrogation signal,” having the highest RSSI while refraining fromresponding to the interrogation signals having a lower RSSI. Thus, thetiming of the tag signal is controlled based on the timing of thestrongest interrogation signal, and other communication characteristicsof the tag signal may be controlled based on the highest interrogationsignal such that the tag signal is successfully received by the readerthat originally transmitted the strongest interrogation signal.

Note that it may be desirable for at least some tags 122 to be permittedto unlock the door 42 from the corresponding restricted area 13. As anexample, it may be desirable for a tag 122 that is attached to a largepiece of equipment to be able to unlock the door 42 from both sides ofthe door 42, whereas other tags 122, such as tags 122 attached tosmaller pieces of equipment, may be allowed to unlock the door 42 onlyfrom the side. To enable a tag 122 to unlock the door 42 from therestricted area 13, the location data 121 may be defined such thatinterrogation-enabled zone 213 is extended into the restricted area 13as may be desired. Thus, when the tag 122 is within the restricted area13 and the interrogation-enabled zone 213, the tag is 122 enabled torespond to the interrogation signals from the reader 26 and, thus,transmit tag signals to such reader 26, thereby initiating an unlockingof the door 42 from the restricted area 13.

In addition, if desired, the tag logic 152 may be configured todetermine whether it is within an interrogation-enabled zone withoutcommunicating with the server 120. As an example, as described above,the tag logic 152 may be configured to track its location based on thereceived signal strengths of signals from the nodes 101-104 orotherwise. The location data 121 may also be stored in the tag 122 suchthat the tag logic 152 is aware of the areas that constituteinterrogation enabled-zones. In such case, the tag logic 152 can compareits location to the location data 121 to determine whether it is in aninterrogation-enabled zone without consulting with or receivinginformation from the server 120.

In one exemplary embodiment, the tag logic 152 or the server 120 isconfigured to predict in which restricted area the user of the tag 122is interested in accessing based on the tag's movements or location. Thetag logic 152 may then control the tag 122 such that it responds tointerrogation signals from the reader corresponding to the area ofinterest without responding to interrogation signals from other readers.

As an example, the server 120 may be configured to determine the speedor velocity of the user as he is walking based on changes in the tag'slocation over time. If the tag 122 approaches a reader without slowingdown, the server 120 may assume that the user is not interested inaccessing the restricted area corresponding to such reader and, thus,refrain from enabling the tag 122 to respond to the interrogationsignals from the reader. However, if the tag's movement begins to slowas it approaches a reader, the server 120 may assume that the user maybe interested in accessing the restricted area corresponding to suchreader and, thus, enable the tag 122 to respond to interrogation signalsfrom the reader. In such embodiment, the tag's speed may be determinedby tracking changes in the tag's location. Also, the tag 122 may beequipped with a motion sensor 189 (FIG. 5), such as an accelerometer,for sensing the tag's motion, and such motion sensor 189 may be used todetermine the tag's speed. Further, the tag logic 152 rather than theserver 120 may determine whether it is to respond to interrogationsignals. As an example, the tag logic 152 may be configured to estimatethe tag's speed based on an accelerometer and to refrain from respondingto interrogation signals until the tag's estimated speed falls below apredefined threshold.

To better illustrate the foregoing, assume that the user of the tag 122desires to access the restricted area 14. Also assume that the server120 is configured to track movement of the tag 122 over time and toenable the tag 122 to respond only when it is in aninterrogation-enabled zone for the tag 122. Further assume that the tag122 enters the interrogation-enabled zone 213 associated with the reader26 as the user passes within close proximity to the reader 26. At suchtime, the user's speed is not likely to decrease significantly since hisor her destination is the restricted area 14. When the tag 122 entersthe interrogation-enabled zone 213, the server 120 would normally enablethe tag 122 to respond to an interrogation signals such that the tag 122would likely respond to interrogation signal from the reader 26.However, before enabling the tag 122, the server 120 is configured toanalyze the tag's speed and to determine whether its speed hassignificantly decreased (e.g., a measured speed has fallen belowthreshold or a measured deceleration exceeds a threshold) within apredefined distance of the reader 26. If so, the server 120 may assumethat user intends to access restricted area 13 and, thus, enable the tag122 to respond to interrogation messages.

However, in the instant example, the user is actually interested in therestricted area 14 and continues walking at about the same rate of speedas he or she passes the reader 26. In such case, the server 120determines that the tag's speed has not significantly decreased withinthe predefined distance of the reader 26 and, thus, refrains fromenabling the tag 122 even though it has entered theinterrogation-enabled zone 213 associated with the reader 26. Since thetag 122 is not enabled by the server 120, the tag 122 does not respondto interrogation signals from the reader 26 as it passes.

Once the user approaches the reader 27, however, the user may begin toslow since he or she intends to open the door 43 and enter therestricted area 14. When the server 120 determines that the tag 122 hasentered the interrogation-enabled zone 214 of the reader 27, the server120 determines whether to enable the tag 122 based on the tag's speed.In the instant example, the server 120 determines that the tag's speedhas significantly decreased within a predefined distance of the reader27 and, thus, assumes that the user is interested in accessing therestricted area 14 corresponding to such reader 27. Accordingly, theserver 120 transmits to the tag 122 a message for enabling the tag 122.Thus, when the tag 122 receives an interrogation signal from the reader27, the tag 122 responds causing the reader 27 to transition the lockingmechanism 33 to an unlocked state, thereby allowing the user to accessthe restricted area 14.

In other embodiments, other techniques may be used for determiningwhether to enable the tag 122. As an example, as the user is approachingthe reader 26, the speeds or velocities measured for the tag 122 mayindicate that the user is turning, such as turning to walk toward therestricted area 14 rather than attempt to access the restricted area 13.Based on such input, the server 120 may determine that the user is notlikely interested in accessing the restricted area 13 and, thus, refrainfrom enabling the tag 122 when the tag is in the interrogation-enabledzone 213 associated with the reader 26 or is otherwise close to thereader 26. Accordingly, the tag 122 is prevented from wasting power byattempting to respond to readers corresponding to restricted areas forwhich the user is not interested. In addition, inadvertent unlocking ofdoors to restricted areas that are not of interest to the user can alsobe prevented.

In one exemplary embodiment, the tag logic 152 is configured to monitorthe rate of change of the signal strength of interrogation signals fromthe same reader in order to determine whether to respond to suchsignals. This rate of change is indicative of the user's speed as he orshe is approaching or passing by the reader. If such rate of change doesnot indicate that the user is slowing while the interrogation signalsare being received, then the tag logic 152 assumes that the user is notinterested in accessing the restricted area corresponding to the readerand, therefore, refrains from responding to the interrogation signals.However, if the rate of change indicates that the user is slowing whileinterrogation signals are being received, then the tag logic 152 assumesthat the user is interested in accessing the restricted areacorresponding to the reader and, therefore, responds to theinterrogation signals by transmitting at least one tag signal to thereader.

To better illustrate the foregoing, assume that the tag 122 begins toreceive interrogation signals from the reader 26 as it is approachingsuch reader 26. As described above, the communication module 175 of thetag 122 measures the received signal strength of the interrogationsignals and provides the tag logic 152 a value (RSSI) indicative of thereceived signal strength. When the RSSI exceeds a predefined thresholdindicating that the tag 122 is within a certain distance of the reader26, the tag logic 152 begins to monitor the rate of change of the RSSIfor the purpose of determining whether to respond to the interrogationsignals.

If the rate of change of the RSSI over some predefined time period(e.g., one second) is above a predefined threshold, indicating that thespeed of the tag 122 is above a threshold, then the tag logic 152 isconfigured to refrain from responding to the interrogation signals. Thatis, the tag logic 152 does not transmit tag signals in response to theinterrogation signals. However, if the rate of change of the RSSIexceeds the predefined threshold, indicating that the speed of the tag122 is below a threshold, then the tag logic 152 responds to theinterrogation signals by transmitting at least one tag signal, which ifsuccessfully received by the reader 26 would initiate an unlocking ofthe door 42.

In addition, the stability of the rate of change in the RSSI may be usedto determine when to respond to the interrogation signals. In thisregard, as the user of the tag 122 is walking towards the reader 23, theRSSI of the interrogation signals from the reader 26 is likely toincrease to an apex value, which would be measured when the tag 122 isapproximately directly in front of the reader 26. If the tag 122continues to move past the reader 26, then the RSSI should begin todecrease. If the tag 122 transmits a tag signal merely because itreceives an interrogation signal from the reader 26, it could result ina security breach by unlocking the door 42 merely because the tag 122passes by the reader 26 without the user intending to access therestricted area 13.

In one exemplary embodiment, the tag logic 152 refrains fromtransmitting a tag signal unless and until the RSSI becomes stable, suchas when the user of the tag 122 is standing in front of the reader 26 ordoor 42 waiting for the door 42 to be unlocked. There are varioustechniques that can be used to determine when the rate of change of theRSSI is stable. In one exemplary embodiment, the tag logic 152determines that the rate of change of the RSSI is stable when:(RSSI(t)−RSS(t+n))/RSSI(t)<X, where RSSI(t) is a measurement of RSSI,RSSI(t+n) is a measurement of RSSI after a predefined time from themeasurement of RSSI(t), and X is a predefined threshold. By waiting forthe rate of change in RSSI to stabilize before transmitting tag signalsto the reader 26, a level of security can be added so that a passing tag122 does not automatically initiate an unlock of the door 42 when theuser of the tag 122 is not likely interested in accessing thecorresponding restricted area 13.

In another exemplary embodiment, the tag 122 may be configured torespond to only certain interrogation signals. In this regard, eachreader 25-27 may transmit an interrogation signal that is uniquerelative to interrogation signals transmitted by other readers. As anexample, the interrogation signal may define an identifier, referred tohereafter as “reader identifier,” that uniquely identifies thetransmitting reader relative to the other readers. The reader data 199may indicate the identifiers of the readers 25-27 for which the tag 122is enabled to respond. As an example, the tag 122 may be configured torespond only to readers 25-27 corresponding to restricted areas forwhich the user of the tag 122 is authorized to access. There are varioustechniques that can be used to control to which readers the tag 122 isresponsive.

In one exemplary embodiment, the reader data 199 is used for thispurpose. Specifically, the reader data 199 is defined to indicate towhich readers 25-27 the tag 122 should respond. As an example, assumethat the user of the tag 122 is only authorized to access restrictedarea 13 such that only reader 26 recognizes the tag's identifier asvalid. In such case, the reader data 199 may be defined to indicate thatthe tag 122 should respond to interrogation signals from the reader 26but not from readers 25 and 27. As an example, the reader data 199 maybe defined to include the reader identifier of reader 26 and not thereader identifiers of readers 25 and 27. When the tag 122 receives aninterrogation signal, the tag logic 152 is configured to compare thereader identifier of such signal to the reader identifiers in the readerdata 199. If there is a match, then the tag logic 152 determines that itis enabled to respond to the interrogation signal (assuming that thatthe tag 122 is not otherwise disabled, such as being outside of aninterrogation-enabled zone) and, therefore, responds to theinterrogation signal by transmitting a tag signal, as described above.However, if there is not a match, then the tag logic 152 determines thatit is disabled from responding to the interrogation signal and,therefore, refrains from transmitting a tag signal. Thus, in the instantexample in which the data 199 only includes the reader identifier ofreader 26, the tag logic 152 should respond to an interrogation signalfrom this reader 26 and should refrain from responding to interrogationsignals from the readers 25 and 27. As a result, the tag 122 does notwaste power attempting to respond to interrogation signals from readerscorresponding to restricted areas for which the user of the tag 122 isnot authorized to access.

In various embodiments described above, the tag 122 is described astransmitting its tag identifier in response to interrogation signals. Itshould be noted that the same tag identifier may be transmitted eachtime that the tag 122 responds to an interrogation signal. However, ifdesired, the tag 122 may be configured to transmit different tagidentifiers depending on which reader transmitted the interrogationsignal to which the tag 122 is responding.

As an example, two different readers may be associated with twodifferent identification systems such that one reader is designed tooperate in conjunction with one conventional passive tag 22 and anotherreader is designed to operate in conjunction with a different passivetag 22. In such case, the same user may be required to carry two tags(each responding with a different tag identifier) in order tointeroperate with both readers. In addition, one reader may havedifferent communication characteristics (e.g., frequency window, timewindow, and strength window) relative to another reader. The tag 122 maybe configured to distinguish interrogation signals from differentreaders and to change its communication characteristics based on whichinterrogation signal it is receiving. The tag 122 may also change whichtag identifier is transmitted based on the interrogation signal.

To better illustrate the foregoing, assume that the reader 26 hasdifferent communication characteristics relative to reader 27. Throughtechniques described above, the tag logic 152 may learn or otherwiseknow the communication characteristics of the reader 26 and separatelylearn or know the communication characteristics of the reader 27. Whenthe tag 122 receives an interrogation signal, the tag logic 152 maydetermine whether the signal is received from the reader 26 or thereader 27. For example, if the interrogation signal includes a readeridentifier, such reader identifier may be used to identify the readerfrom which the interrogation signal originates. However, in otherembodiments, other techniques may be used to determine which reader 26or 27 transmitted the interrogation signal. For example, if the reader26 transmits at a different frequency relative to the reader 27, the taglogic 152 may distinguish the interrogation signals based on frequency.Also, the tag's location may be used to distinguish interrogationsignals.

In this regard, as described above, the tag logic 152 or server 120 maytrack the tag's location as it travels. If the tag 122 receives aninterrogation signal while it is in a certain location (e.g., close tothe reader 26), then the tag logic 152 may assume that it is receivingan interrogation signal from the reader 26. In such case, the tag logic152 selects the communication characteristics and/or tag identifierappropriate for the reader 26 in order to respond to the interrogationsignal. Using the selected communication characteristics, the tag 122replies with a tag signal that includes the selected identifier.However, if the tag 122 receives an interrogation signal while it is inanother location (e.g., close to the reader 27), then the tag logic 152may assume that it is receiving an interrogation signal from the reader27. In such case, the tag logic 152 selects the communicationcharacteristics and/or tag identifier appropriate for the reader 27 inorder to respond to the interrogation signal. Thus, the communicationcharacteristics (e.g., timing, frequency, and signal strength) and/orthe tag identifier of the tag signal may be tailored based on whichreader is determined to have transmitted the received interrogationsignal. Note that readers may be grouped together for the purpose ofselecting communication characteristics and/or tag identifiers. As anexample, the readers 25 and 26 may be members of the same identificationsystem. Thus, these readers 25 and 26 may recognize the same tagidentifier and use the same communication characteristics. In such case,the tag logic 152 may be configured to select the same communicationcharacteristics and tag identifier when responding to interrogationsignals from either reader 25 or 26.

In the embodiments described above, various actions are described asbeing performed by the tag logic 152 or the server 120. It should benoted that actions described herein as being performed by the server 120may instead be performed at the tag 122 by the tag logic 152, andactions described herein as being performed by the tag logic 152 may beperformed by the server 120.

An exemplary operation and use of the system 100 will now be describedin more detail below.

For illustrative purposes, assume that the conventional passiveidentification system 10 shown by FIG. 1 is used to control access torestricted areas 12-14 at a facility. Also, assume that a user of thepassive tag 22 is authorized to access restricted area 13. In such case,the tag identifier of the tag 22 is included in the list of valididentifiers stored at the reader 26.

Further assume that it is desirable to migrate the system 10 frompassive to active by using an active tag 122 in lieu of the conventionalpassive tag 22. In such case, the nodes 101-104 are installed, andlocation data 121 defining a map of the facility is stored at the server120. Such location data 121 preferably indicates the locations of thenodes 101-104 and the readers 25-27 on the map. The location data 121also defines the boundary of the interrogation-enabled zone 213. Inaddition, the tag identifier of the passive tag 22 is preferably storedin the active tag 122 and used by the tag 122 to request access torestricted areas, as will be described in more detail below.

Initially, the tag 122 is outside of the interrogation-enabled zone 213corresponding to the reader 26 and, thus, is disabled from responding tointerrogation signals. Assume that the user walks toward the door 42 forthe purpose of entering the restricted area 13. As the user walks, thetag's location is tracked by the server 120. Once the user moves the tag122 close enough to the reader 26 such that it enters theinterrogation-enabled zone 213 corresponding to the reader 26, theserver 120 transmits a message for enabling the tag 122 such that itresponds to interrogation signals.

Thereafter, the tag logic 152 responds to an interrogation signal fromthe reader 26 by transmitting at least one tag signal that indicates thetag identifier previously transmitted by the passive tag 22 that hasbeen now replaced by the active tag 122. As an example, the tag logic152 may first listen to two consecutive interrogation signals from thereader 26 before transmitting a tag signal. Based on the interrogationsignals, the tag logic 152 selects various communication characteristicsfor the tag signal. As an example, the tag logic 152 may select atransmit power for the tag signal based on the received signal strengthof at least one of the interrogation signals. Also, the tag logic 152may select a rate for transmitting tag signals during a response periodand also select a duration of the response period based on the timelapse between the two successive interrogation. Then, upon reception ofthe next interrogation signal, the tag logic 152 transmits via thecommunication module 175 tag signals in accordance with the selectedcommunication characteristics.

Assuming that at least one of the tag signals satisfies the time,frequency, and signal strength windows expected by the reader 26, thereader 26 should successfully read such tag signal to determine thetag's identifier and provide this tag identifier to the controller 36,which compares such tag identifier with the list of valid tagidentifiers stored at the reader 26. In response to a match, thecontroller 36 controls the locking mechanism 32 such that it transitionsto an unlocked state thereby allowing the user to open the door 42 andenter into the restricted area 13.

Accordingly, the conventional identification system 10 using a passivetag 22 is migrated to a system for which an active tag 122 can replacethe passive tag 22 without having to update or change the readers 25-27.Indeed, the migration can be completely transparent to the readers 25-27while not compromising the security of the restricted areas 12-14.

Note that the tag's communication characteristics include the frequency,power, and timing of the tag signals that are transmitted by the tag122. Timing of the tag signals involves the decisions about whether andwhen to transmit tag signals. As described above, the communicationcharacteristics may be selected or otherwise determined based on thetag's location, as determined via communication between the tag 122 andthe nodes 101-104. For example, as described above, the tag's location(e.g., whether the tag 122 is within an interrogation-enabled zone) maybe used to determine whether the tag 122 should respond to aninterrogation signal from a particular reader 25-27. The tag's locationmay also be used to determine the appropriate signal strength or delayfor transmitting a tag signal. Various other types of communicationcharacteristics may be based on the tag's determined location.

For simplicity of illustration, various examples have been describedabove in the context of a single tag 122. It should be emphasized thatthe system 100 may be used to simultaneously monitor and track anynumber of tags 122 according to the techniques described above. In thisregard, it is possible for each tag 122 to be associated with arespective set of location data 121 and reader data 199 so that thecontrol parameters (e.g., which restricted areas 12-14 are accessible bythe respective tag 122) can be tailored for each individual tag 122.

In one exemplary embodiment, the tags 122 are interchangeable such thatthe same tag 122 may be associated with different assets at differenttimes. As an example, a tag 122 may be used by an employee during thecourse of work day and then turned in at the end of his or her shift sothat the tag 122 can be used by another employee. Such feature allowsmultiple employees to share the same tag 122. This feature also allowsthe tags 122 to be returned to a central location where the tags can berecharged and then distributed to employees without the same tag 122having to be returned to the same employee.

To allow the same tag 122 to be used by a different employee, it maydesirable to update the control parameters of the tag 122 so that theyare tailored to the tag's current user. To achieve this, each set ofreader data 199 and location data 121 for a particular asset (e.g.,employee) is associated with a unique identifier, referred to hereafteras “asset identifier.” Thus, the asset identifier may be used as a keyto lookup and find the set of reader data 199 and location data 121 thatare associated with a particular asset.

When a tag 122 is to be used to track and monitor a particular asset,the asset identifier for this asset is preferably input to the tag 122or otherwise received by the system 100. As an example, the assetidentifier may be input via the input interface 181 of the tag 122. Insuch case, the tag logic 152 transmits the asset identifier to theserver 120, which may be configured to store the sets of location data121 and reader data 199 for all of the assets to be tracked andmonitored by the system 100. In response, the server 120 retrieves theset of location data 121 and the set of reader data 199 associated withthe asset identifier and downloads these sets of data 121 and 199 to thetag 122. These sets of location data 121 and reader data 199 are thenused to control the tag 122 according to the techniques described above.For example, as described above, the tag 122 may be enabled to respondto interrogation signals from only certain readers based on the locationdata 121 and/or reader data 199. Note that the tag identifier that isused to access restricted areas 12-14 may be included in the reader data199 that is downloaded to the tag 122.

At some point the same tag 122 may be coupled to a different asset. Forexample, an employee wearing the tag 122 may turn in the tag 122 at theend of his or her shift, and the tag 122 may be given to anotheremployee for use in tracking and monitoring this other employee. Eachtime the tag 122 is transferred to a new asset, the tag 122 ispreferably re-commissioned for use with the new asset. In this regard,the aforementioned process of downloading location data 121 and readerdata 199 is repeated for the new asset such that the tracking andmonitoring of the tag 122 is now based on the location data 121 andreader data 199 associated with the asset identifier of the new asset.Thus, which restricted areas 12-14 are accessible via the tag 122 andthe tag's responsiveness to the readers 25-27 may be different for thenew asset relative to the previous asset. In this regard, the new readerdata 199 downloaded to the tag 122 for use with the new asset includes anew tag identifier that is thereafter transmitted by the tag 122 inresponse to interrogation signals. Using a different tag identifier canchange which of the controllers 35-37 recognize the tag 122 for thepurposes of granting access to restricted areas. Also, the new set oflocation data 121 may define different interrogation-enabled zones forthe tag 122 such that the tag 122 is responsive to a different set ofreaders 25-27 for the purpose of transmitting tag signals. Accordingly,by using a new set of location data 121 and reader data 199 to controlthe tag 122 for a new user, the operational characteristics of the tag122 and the system 100 can be tailored for such new user and changedfrom user-to-user.

To facilitate the entry of asset identifiers during operation, eachasset may have or otherwise be associated with a scannableidentification card, which can be scanned by the input interface 181 ofthe tag 122 in order to input the appropriate asset identifier for theasset. As an example, the identification card may have a barcodeindicating an asset identifier that can be automatically read by theinput interface 181 of the tag 122. By simply scanning theidentification card, the tag 122 receives the asset identifier and usessuch identifier to automatically adopt the unique control profileassociated with the identified asset. Such adoption of new profilesallows tags 122 to be used interchangeably with different users who maybe associated with different profiles, as described above. For example,a tag 122 used by one employee may be returned to a central tagrepository at the end of the employee's shift, charged overnight, andcommissioned for use with another employee the next morning. The abilityto use the same tag 122 for multiple assets can reduce the overallnumber of tags 122 needed for operation and allow central and completecharging of the tags 122 from time-to-time, as well as maintenanceperformed on any one tag 122 without preventing the tag's user fromaccessing restricted areas.

Now, therefore, the following is claimed:
 1. An active tag for emulatingpassive tags when communicating with readers of a passive-tagidentification system, the readers configured to wirelessly transmitinterrogation signals for interrogating passive tags within thepassive-tag identification system, the passive tags configured towirelessly transmit tag signals in response to the interrogationsignals, the tag signals including tag identifiers for identifying thepassive tags, the readers including at least a first reader and a secondreader, the active tag comprising: a communication module forcommunicating with the readers, the communication module having areceiver and a transmitter, the receiver configured to receive a firstinterrogation signal from the first reader and a second interrogationsignal from the second reader; a power supply for powering components ofthe active tag, including at least the transmitter; and logic configuredto control the transmitter such that the transmitter wirelesslytransmits a first tag signal in response to the first interrogationsignal, the logic configured to determine based on the firstinterrogation signal at least one of an identifier of the first readerand a frequency of the first interrogation signal, the logic configuredto select a first frequency for the first tag signal based on the atleast one of the identifier of the first reader and the frequency of thefirst interrogation signal, the logic further configured to control thetransmitter such that the first tag signal is transmitted at theselected first frequency, wherein the first tag signal transmitted atthe selected first frequency emulates a transmission from a passive tagfor responding to the first interrogation signal, wherein the logic isconfigured to control the transmitter such that the transmitterwirelessly transmits a second tag signal in response to the secondinterrogation signal, wherein the logic is configured to determine basedon the second interrogation signal at least one of an identifier of thesecond reader and a frequency of the second interrogation signal,wherein the logic is configured to select a second frequency for thesecond tag signal based on the at least one of the identifier of thesecond reader and the frequency of the second interrogation signal,wherein the logic is further configured to control the transmitter suchthat the second tag signal is transmitted at the selected secondfrequency, and wherein the second tag signal transmitted at the selectedsecond frequency emulates a transmission from a passive tag forresponding to the second interrogation signal.
 2. The active tag ofclaim 1, wherein the logic is configured to communicate with a pluralityof nodes of a wireless network of an asset tracking system fordetermining a location of the active tag.
 3. The active tag of claim 2,wherein the logic is configured to determine whether to transmit thefirst tag signal in response to the first interrogation signal based onthe determined location.
 4. The active tag of claim 2, wherein the logicis further configured to select the first frequency for the first tagsignal based on the determined location.
 5. A method for emulatingpassive tags when communicating with readers of a passive-tagidentification system, the readers configured to wirelessly transmitinterrogation signals for interrogating passive tags within thepassive-tag identification system, the passive tags configured towirelessly transmit tag signals in response to the interrogationsignals, the tag signals including tag identifiers for identifying thepassive tags, the readers including at least a first reader and a secondreader, the method comprising: receiving a first interrogation signalfrom the first reader at an active tag; receiving a second interrogationsignal from the second reader at the active tag; determining with theactive tag based on the first interrogation signal at least one of anidentifier of the first reader and a frequency of the firstinterrogation signal; determining with the active tag based on thesecond interrogation signal at least one of an identifier of the secondreader and a frequency of the second interrogation signal; selectingwith the active tag a first frequency for a first tag signal based onthe at least one of the identifier of the first reader and the frequencyof the first interrogation signal; selecting with the active tag asecond frequency for a second tag signal based on the at least one ofthe identifier of the second reader and the frequency of the secondinterrogation signal; transmitting the first tag signal from the activetag to the first reader at the selected first frequency in response tothe first interrogation signal, wherein the first tag signal transmittedat the selected first frequency emulates a transmission from a passivetag for responding to the first interrogation signal; and transmittingthe second tag signal from the active tag to the second reader at theselected second frequency in response to the second interrogationsignal, wherein the second tag signal transmitted at the selected secondfrequency emulates a transmission form a passive tag for responding tothe second interrogation signal.
 6. The method of claim 5, furthercomprising: wirelessly communicating between the active tag and aplurality of nodes of a wireless network; and determining a location ofthe active tag based on the communicating between the active tag and theplurality of nodes.
 7. The method of claim 6, further comprising:storing, in memory, data indicating a plurality of interrogation-enabledzones for the active tag; determining, based on the data and thedetermining the location of the active tag, whether the active tag iswithin one of the interrogation-enabled zones; and determining whetherto transmit the first tag signal from the active tag based on whetherthe active tag is determined to be within at least one of theinterrogation-enabled zones.
 8. An active tag for emulating passive tagswhen communicating with readers of a passive-tag identification system,the readers configured to wirelessly transmit interrogation signals forinterrogating passive tags within the passive-tag identification system,the passive tags configured to wirelessly transmit tag signals inresponse to the interrogation signals, the tag signals including tagidentifiers for identifying the passive tags, the active tag comprising:a communication module having a transmitter and a receiver forcommunicating with the readers, the receiver configured to receive aninterrogation signal from one of the readers; a power supply forpowering components of the active tag; and logic configured towirelessly transmit a tag signal via the communication module inresponse to the received interrogation signal, the logic configured todetermine a first communication characteristic and a secondcommunication characteristic of the tag signal based on at least oneinterrogation signal transmitted by the one of the readers such that thetag signal emulates a transmission from a passive tag for responding tothe received interrogation signal, the logic further configured tocontrol the tag signal according to the first communicationcharacteristic and the second communication characteristic determined bythe logic, wherein the first communication characteristic pertains to atiming of the tag signal, wherein the logic is configured to transmitthe tag signal after a predefined delay from reception of theinterrogation signal received from the one of the readers such that theone of the readers receives the tag signal within an expected timewindow for receiving a response to the received interrogation signal,wherein the second communication characteristic pertains to a frequencyof the tag signal, and wherein the logic is configured to transmit thetag signal within an expected frequency window for the one of thereaders to receive the response to the received interrogation signal. 9.A method for emulating passive tags when communicating with readers of apassive-tag identification system, the readers configured to wirelesslytransmit interrogation signals for interrogating passive tags within thepassive-tag identification system, the passive tags configured towirelessly transmit tag signals in response to the interrogationsignals, the tag signals including tag identifiers for identifying thepassive tags, the method comprising: receiving an interrogation signalfrom one of the readers via an active tag; transmitting a tag signalfrom the active tag to the one of the readers in response to thereceived interrogation signal; automatically determining a firstcommunication characteristic and a second communication characteristicof the tag signal with the active tag based on at least oneinterrogation signal transmitted by the one of the readers such that thetag signal emulates a transmission from a passive tag for responding tothe received interrogation signal; and controlling the tag signal withthe active tag according to the first communication characteristic andthe second communication characteristic determined by the active tag,wherein the first communication characteristic pertains to a timing ofthe tag signal, wherein the transmitting the tag signal comprisestransmitting the tag signal after a predefined delay from the receivingsuch that the one of the readers receives the tag signal within anexpected time window for receiving a response to the receivedinterrogation signal, wherein the second communication characteristicpertains to a frequency of the tag signal, and wherein the transmittingthe tag signal comprises transmitting the tag signal within an expectedfrequency window for the one of the readers to receive the response tothe received interrogation signal.
 10. An asset tracking system,comprising: a first reader configured to wirelessly transmitinterrogation signals for interrogating passive tags; a controllerconfigured to control access to a first restricted area, the controllercommunicatively coupled to the first reader; a second reader configuredto wirelessly transmit interrogation signals for interrogating passivetags; a network having a plurality of nodes; an active tag configured towirelessly communicate with the nodes for determining a location of theactive tag, the active tag configured to receive a first interrogationsignal from the first reader and to wirelessly transmit a first tagsignal in response to the first interrogation signal, the first tagsignal having a tag identifier for identifying the active tag, theactive tag configured to determine based on the first interrogationsignal at least one of an identifier of the first reader and a frequencyof the first interrogation signal, the active tag configured to select afirst frequency for the first tag signal based on the at least one ofthe identifier of the first reader and the frequency of the firstinterrogation signal, the active tag further configured to control thefirst tag signal such that the first tag signal is transmitted at theselected first frequency, wherein the first tag signal transmitted atthe selected first frequency emulates a transmission from a passive tagfor responding to the first interrogation signal, wherein the active tagis configured to wirelessly transmit a second tag signal in response tothe second interrogation signal, wherein the active tag is configured todetermine based on the second interrogation signal at least one of anidentifier of the second reader and a frequency of the secondinterrogation signal, wherein the active tag is configured to select asecond frequency for the second tag signal based on the at least one ofthe identifier of the second reader and the frequency of the secondinterrogation signal, wherein the active tag is further configured tocontrol the second tag signal such that the second tag signal istransmitted at the selected second frequency, and wherein the second tagsignal transmitted at the selected second frequency emulates atransmission from a passive tag for responding to the secondinterrogation signal, and wherein the first reader is configured toreceive the first tag signal and to provide the tag identifier to thecontroller for authorizing access to the first restricted area inresponse to the first tag signal.
 11. The system of claim 10, whereinthe active tag is configured to determine whether to transmit the firsttag signal in response to the first interrogation signal based on thedetermined location.
 12. The system of claim 10, further comprising aserver configured to determine the location of the active tag and tocontrol, based on the location, whether the active tag transmits thefirst tag signal in response to the first interrogation signal.
 13. Thesystem of claim 12, wherein the server is configured to store data inmemory, the data indicating a plurality of interrogation-enabled zonesfor the active tag, wherein the server is configured to determinewhether the active tag is in at least one of the interrogation-enabledzones based on the location, and wherein the server is configured tocontrol whether the active tag transmits the first tag signal inresponse to the first interrogation signal based on the whether theactive tag is determined to be within at least one of theinterrogation-enabled zones.